1. Field of the Invention
This invention relates to a driving method for a stepping motor and a sheet conveying apparatus using the driving method.
More particularly, the present invention relates to a driving method for a stepping motor whereby a predetermined amount of feed of the stepping motor in a system in which, for example, the time interval of the generation of a drive command (a sheet conveyance command or the like) is not constant is driven by a predetermined number of pulse trains per said predetermined amount, and a sheet conveying apparatus using such method.
2. Related Background Art
There are numerous examples of a paper conveying systems in facsimile apparatus in which a stepping motor is used to convey paper. In one such example the line feed corresponding to one line of printed dots is accomplished by a plurality of steps (such as, for example, two steps/line or four steps/line) of the stepping motor, and heretofore, the step drive interval during the line feed (i.e., the pulse rate) has been fixed.
Now, in the case of the image transmission, for example, by G3 facsimile, the period of generation of a line feed command depends on the processing time for MH-coding or MR-coding line data read by a scanner. That is, in a complicated image portion, the run length is short and therefore a long time is required for the coding and thus, the period of generation of the line feed command becomes long. However, in a simple image portion, the run length is long and the coding thereof can be accomplished within a short time. Accordingly, the period of generation of the line feed command also becomes short. For this reason, the pulse rate of the conventional stepping motor has been set on the basis of the minimum period of generation of the line feed command, and is fixed.
FIG. 1 of the accompanying drawings is a waveform graph showing the axial displacement D'.sub.A of a stepping motor by the conventional stepping motor driving method.
In FIG. 1, a line advance signal LA (a line feed command) is shown as varying for each line feed. Accordingly, the interval between the points of variation shows the periods of generation A-D of the line feed command. Also, a drive pulse signal DP is set correspondingly to the minimum period of generation A of the line feed command, and the pulse rate is fixed by the relation between this period A and the four steps/line. Here, the pulse rate can be found from ##EQU1## Accordingly, if the minimum period of generation A is long, the pulse rate becomes small. The rotor is axially displaced for each pulse and moves along an ideal axial displacement signal IAD. However, if for example, the minimum period of generation A is as short as 5 ms and is still driven by four steps/line, the pulse rate is 800 pps, which means a very high speed. When the stepping motor is driven at such a pulse rate, overshoot or the undershoot from the influence of the inertia of the rotor occurs, and this often leads to the undesirable phenomenon of disordered condition of the rotor by damping.
That is, in the sections of the periods of generations A and B of FIG. 1, eight drive pulse signals DP continue and therefore, the rotor is considerably accelerated, and in the latter half of the period of generation B, the overshoot as shown by an axial displacement signal AAD occurs due to the moment of inertia of the rotor. The rotor then tries to return to a prescribed position b. This is the so-called phenomenon of damping. If at that time, the period of generation B is in the relation as shown in FIG. 1, the drive pulse signal DP generated at the beginning of the next period of generation C will flatly receive the return torque of the rotor and the driving energy may be offset thereby. Accordingly, although the axial displacement D'.sub.A of the rotor should originally reach the point of the axial displacement c within the section of the period of generation C, it stops at the point of the axial displacement b or at a point displaced a little therefrom. Thus, there has heretofore been the undesirable possibility that the phenomenon of disordered condition of the rotor is caused.